In general, the present invention relates to the prevention or reduction of wind suction forces induced on the roof of a flat top building, or only slightly inclined roof, generally less than a 40% grade, due to incident high winds. More particularly, the invention relates to unique rooftop structures and method for mitigating wind suction using an associated novel structural protuberance that extends at least partially into the shear layer/transition layer of the flow, whether permanently fixed to the roof top by suitable means, partially or fully embedding or otherwise integrating within the roof by molding, forming, setting, etc. These novel structures reduce or may eliminate the amplification of pressure drops caused by wind gusts flowing over the rooftop.
Windstorm related losses average several billion dollars annually. Roof covering failure, in particular, is a widespread type of damage observed after hurricanes. Once an area of the roof is damaged, building and home interiors are exposed to further damage from inclement weather. The focus of concern, here, is the damage caused to flat top or shallow pitched roofs of buildings due to high winds associated with a storm regardless of the particular meteorological designation of the storm. High winds cause unwanted roof suctions that can severely damage or completely destroy the roof as well as the building structure. More recent studies indicate that the worst mean and peak suctions on flat building roofs occur for xe2x80x98corneringxe2x80x99 or xe2x80x98obliquexe2x80x99 wind angles which are those wind components directed toward any corner of the building where roof-wall junction is xe2x80x98sharpxe2x80x99, i.e., incident winds directed over a range, in FIG. 1A labeled 14A, 14B with a representative angle, xcex1, from approx. 25xc2x0 on either side of the central direction represented by arrow 12xe2x80x94for further reference see Attachment A, Banks, D. (spring 2000), as well as FIG. 4B illustrating incident cornering wind characteristics. As one can see, for cornering or oblique wind angles, conical-shaped vortices, also called delta wing vortices, form along the roof edges. For incident winds directed generally normal, or perpendicular, to a wall of the building with no significant cornering component, i.e., those incident winds directed over a range, in FIG. 2A labeled 34A, 34B with a representative angle, xcex2, from approx. 20xc2x0 on either side of the direction represented by the central arrow 32xe2x80x94see also FIGS. 2B, 3A-3B, 4A, 5A-5B, and 6xe2x80x94the vortex induced suction is generally not as destructive as vortices formed during cornering winds. While previous studies have attempted to progress toward linking wind flow characteristics to surface pressures, prior to the instant analysis, the mechanism linking vortex structure and roof surface pressure has been little understood. The rigorous analysis performed and resulting dynamic link between vortex behavior, surface pressures, and wind flow characteristics as identified herein, have led to the ingenious structures of the invention. Based upon the work of the applicants, comparison of simultaneously recorded image data of a rooftop corner vortex and pressures therebeneath indicate and confirm that the peak suction lies beneath the vortex core, and moves with the vortex. An explanation of the analysis and experimentation performed by applicants is found in Attachment A, Banks, D. (spring 2000), and Attachment B, Wu, F., excerpts from dissertation, Chapter 8xe2x80x94both of which are also identified below and are incorporated herein by reference.
The greatest force on the building is known to be the uplift on the roof, and this is a very common failure mode. The worst suction on both gabled and flat roofs are known to occur beneath the vortices that form in the separated flow along the roof edges. For the flow considered generally normal to a wall, FIG. 2A within the range defined by xcex2, a condition known as xe2x80x9cbubble separationxe2x80x9d predominates within which the vortices are formed, see also FIGS. 1C, 3A-4A, and more-particularly, FIG. 2.12 on page 67 of Attachment A. In this situation, the position of the reattachment varies considerably, and is considered unstable, and the vortices which form along the roof edge in the separated flow form and are convected away from the edge at irregular intervals. For reference, see FIG. 1C where reattachment at the rooftop 10 occurs at 29. In contrast, for cornering flow, the flow separation on flat or gabled roofs takes the form of stable dual conical vortices. Thus, it remains to more closely examine the flow mechanism by which a vortex instantaneously controls rooftop suctions. By focusing on understanding the vortex behavior as it is connected to rooftop suctions, the unusual corresponding pressure characteristics may be more fully examined. To do this, a novel analytical model for vortex pressure field was developed and assessed experimentally. This new model quantifies how two parameters, streamline curvature and flow speed above the vortex, control surface pressure. Experimental data confirms that the model accounts for changes in surface pressure with wind direction and proximity to the roof corner. Finally, the model suggests that by inhibiting the flow reattachment, the effect of the vortices on the roof can be by and large, eliminated.
Turning to the two-dimensional schematic xe2x80x98snap-shotsxe2x80x99 of FIGS. 1C and 3A-3B, one can better appreciate the dynamics vortex flow model of the invention: Within the xe2x80x9ctransition region/layerxe2x80x9d (TR) the velocity of the fluid (for example, air) is higher than that of the fluid on the same streamline, upstream, due to the well known fluid mechanics concept of the xe2x80x9ccontinuity equationxe2x80x9d. The continuity equation embodies the concept of conservation of mass, and as applied to the situation here, one can appreciate that air. behaving essentially as an incompressible fluid, speeds up as it passes over the roof-edge of a building. Boundary layer theory dictates that the flow speed right at the roof surface must be zero so that the flow speed in the transition region decreases rapidly toward the roof. This results in shear stress and vorticity within the fluid flow so that one can make the correlation that the transition region/layer roughly corresponds to a xe2x80x98shear layerxe2x80x99.
In the normal wind condition, the region of slow or re-circulating flow under the transition or shear layer is called the separation region, or, separation bubble. xe2x80x9cReattachmentxe2x80x9d of the flow is defined to occur at the xe2x80x98endxe2x80x99 of the separation region and is the point/area at which the flow returns to traveling generally parallel to the roof surface, once again, easier seen in FIG. 1C at 29. As one can see, in the normal incident wind case, the separation region encompasses the vortices as well as an area surrounding the vortices. Ideally, the preferred structures of an apparatus of the invention are positioned and affixed to disrupt, or, xe2x80x98catchxe2x80x99/separate, the flow within the on coming flow""s transition region (TR) such that this point of reattachment (e.g., 29) is moved further out and away from the roof-wall edge, toward the right in FIG. 1C.
In wind engineering research, interest in understanding roof corner vortices is high not only because of the direct correlation to high roof suction, but because of several peculiarities observed during pressure measurement:
1) The discrepancy between full-scale and model-scale peak pressuresxe2x80x94while the results of scaled model studies and full-scale test provide matching mean pressure coefficients over the whole building, the peak and root mean square (rms) pressure coefficients do not match under the separated flow, where the vortices are located. There, the full-scale rms and peak suctions are higher for the full-scale tests. This is a concern, since the building codes of many countries are based upon scaled-model tests in boundary layer wind tunnels.
2) The quasi-stead theory is often used in building codes to predict peak pressures based upon knowledge of mean pressure coefficients and of the turbulence characteristics of the upstream flow. The quasi-steady theory is generally fairly accurate for most of the building, but does not work well for pressures beneath the separated flow.
3) Taps beneath the vortices have exhibited bi-modal probability distributions. This is not generally seen anywhere else on the structure.
4) Extreme peak pressures beneath the vortices are better correlated along the length of the vortex than velocity gusts in the upstream flow, or pressures elsewhere on the buildingxe2x80x94the result of the presence of a coherent flow structure on the building roof (the vortex).
Prior attempts by other researchers to understand how upstream flow conditions could control rooftop surface pressures reveals that the debate over the discrepancies identified immediately above, is really subordinate to the question of explaining how the extremely low pressures near the roof edge occur. Until applicants"" rigorous analysis and identification or their solution, no acceptable explanation had been offered for the existence and occurrence of these extremely low pressures.
While it is known that replacing a sharp building edge with a curved roof-wall edge plays a part in disrupting the creation of unwanted rooftop vortices due to incident winds, see FIG. 1 on page 160 of Richardson and Surry, as does securing a tall enough solid parapet flush with the building outside wall, see for reference, FIG. 8B, around the full periphery of the roof, these are impractical and undesired architectural solutions due to additional cost as well as design and aesthetic considerations, and to the problem of building and roof damage and destruction due to high winds. Other than these designs, prior solutions suggested to minimize roof damage include: super-reinforced joints/edges, and a membrane overlying a deck with an air permeable and resilient mat installed over the membrane.
Therefore, new useful structures and methods are needed for mitigating the uplift effect caused by vortices created due to incident high winds atop flat or slightly inclined roof surfaces. Unlike conventional systems and solutions, the innovative apparatus and associated technique for mitigating, and under certain circumstances, eliminating the vortices in the separated/re-circulation flow zone/region, is a more-effective, less costly tool for doing so. In the spirit of design goals and related system analysis contemplated hereby, many different types of materials, securing and mounting mechanisms, self-deployment assemblies, and associated structures apply, as will be further appreciated.
It is a primary object of this invention to provide a rooftop apparatus for mitigating wind suction forces induced on a generally flat top, or slightly inclined, generally less than a 40% grade, rooftop due to incident high winds, whether cornering or normal. The focus is to provide unique rooftop apparatus structures and a corresponding technique comprising novel structural protuberances that extend at least partially into the shear layer/transition layer as identified here, to separate the flow therein. The structures need not be large, and need not extend the whole of the periphery of the roof edge. But rather, the novel smaller-sized sturdy structures of the invention are positioned and suitably anchored/affixed/mounted/integrated with a rooftop, wall, frame, the ground, etc. in proximity to one or more of the corners of the building or any other area of interest to reduce amplification of rooftop pressure drops, thus, reducing the high peak suctions experienced beneath separated flows atop the roof.
The advantages of providing the new apparatus and technique of the invention include: simplicity of design and installation; case of adaptation to the wide variety of flat and slightly inclined rooftop and building designsxe2x80x94providing additional design tools to architects and structural engineers in creating new plans, remodeling existing structures, and accommodating design flaws in either; plus a reduction in overall cost to fabricate and employ as partial or full solutions to rooftop and related building damage due to incident high-winds. Further, these and other advantages plus a better understanding of the very distinguishing features of the instant invention, as described and supported by this disclosure, will be readily appreciated in connection with reviewing the attachment excerpts and drawings, as well as the specification and claims.
Briefly described, once again, the invention includes an apparatus secured to extend upwardly from a rooftop for redirecting free oncoming flow of a gas passing over an edge of the rooftop. The apparatus has an elongated member having upper and lower flow-surfaces, and a leading edge portion having a leading-rim extending therealong. A plurality of supports is spaced along the elongated member. An upper-end of each support is secured to, integrated-with, etc., or otherwise extends from the lower flow-surface of the elongated member. The lower-ends of each support can be mounted, integrated-with/built into, or otherwise suitably fastened or secured directly to the rooftop, integrated-with/mounted to a substantially rigid elongated horizontal structure which is, in turn, mounted to the rooftop, and so on, so as to extend therefrom providing a spaced relationship between the lower flow-surface and rooftop. The leading-rim of the leading edge portion extends into the oncoming flow to redirect of at least a portion thereof so that it flows under the leading edge portion and along the lower flow-surface.
In the case of heavy winds, especially those experienced with severe weather including tornado and twisters of the Midwest, hurricanes of the Coastal regions, and associated heavy prevailing winds, the free oncoming flow of a gas includes a strong gust of wind, which can be brief or extended/prevailing heavy winds, with a transition flow region. Over the rooftop the transition region of the oncoming flow, labeled xe2x80x9cTRxe2x80x9d for reference in FIGS. 3A and 3B, is generally bounded, below, by a re-circulation region wherein flow of the wind travels in a circuitous motion. This unstable re-circulation region can include a vortex such as those depicted with swirling arrows throughout the drawingsxe2x80x94necessarily in depicting such wind phenomena in still-life schematic views, an instant in time is captured and drawn although in the case of wind gusts, vortices existing within the re-circulation region are quite unstable. Preferably, the elongated member of the invention is mounted/secured/integrated such that its leading-rim extends into at least this transition flow region of the oncoming flow; in most instances, the leading-rim will protrude out over the edge of the rooftop. Critically, in effect, the leading edge portion of the elongated member xe2x80x98catchesxe2x80x99 at least a portion of the oncoming flow so that it can be redirected under and along the lower flow-surface toward and into the re-circulation region; by doing so, the creation of vortices can be prevented, thus, mitigating the strong suction associated with vortices, atop the roof. Rooftop edge, as contemplated and used throughout, is inclusive of the edges of the roof (often referred to as xe2x80x98leading edgesxe2x80x99 in the literature in connection with an oncoming wind gust direction) as well as roof cornersxe2x80x94the total number of which will depend upon number of walls joining at angles.
The cross-section of the elongated member, taken between the leading edge portion and trailing end portion of the elongated member, can be of a number of identifiable shapes such as: an oval, a thin-irregular oval, an airfoil, a triangle, a rectangle, a thin-irregular rectangle, a circle, a thin-plate having a curvilinear leading edge portion, a thin-irregular wave-shape, a polygon, and a thin-irregular polygon, for reference by way of example only, see FIGS. 3A, 5B, 7A-7F, 9, 10, 12A-12D. Thus, as shown, the upper and lower flow-surfaces may each be generally planar, may include one or more curvilinear surface (convex or concave), may include an angle, and so on. Likewise, as shown, the leading edge portion of the elongated member can include an angled surface, e.g., FIG. 3A, one or more curvilinear surface, e.g., FIG. 5B, a flat surface, e.g., FIG. 7B, and so on. Furthermore, the lower flow-surface can be oriented in a decline, e.g., see FIGS. 1C, 7B, 7E, 9, 12B, 12Dxe2x80x94wherein the leading edge portion of the elongated member is located further from the rooftop than that of a trailing end portion thereof.
For reference, a distance, d, that the leading-rim protrudes from a sidewall of a building to which the rooftop is attached, may be between 0.05% and 75% of the width, w, of the elongated member as measured from its leading-rim to its trailing end; and in terms of spacing, s, distance, d, as shown is less than a vertical distance, s, between the rooftop and a closest point of the lower flow-surface to the rooftop. A second elongated member can be joined at an end of the first elongated member in angled positional relationship, such as is shown in FIG. 6 at 60A and 60B and in FIGS. 11A-11C at 20A and 20B, for reference only, in a V-shape. Where the edge of the rooftop includes a corner interposed between a first and second edge-length of the roof (likewise angled), at least two of the supports are preferably mounted to the rooftop along each of the first and second edge-lengths, for stability. Further, as identified at 220 in FIGS. 11A-11B, one or more support can be reinforced by a metal strip fastened to a respective sidewall.
The supports may each include a self-deployment mechanism, for reference, see FIGS. 12A-12D: The elongated member has a first lowered position, which, for certain configurations, may mean at least a portion of its lower flow-surface is in contact with the rooftop, and includes moving parts such that it is adapted for moving into a second position. Here, while in its first lowered position, when a gust of wind sufficiently-strong blows against the elongated member, it is self-deployed to its second operational position. The self-deployment mechanism may comprise a first and second hinged extension. The support upper-end is pivotally secured to the lower flow-surface and the support lower-end is pivotally secured to extend from the rooftop. These hinged extensions when in the lowered position are in a generally folded arrangement and adapted for locking into place upon reaching the second position. Alternatively, the mechanism may comprise a plurality of telescoping sections located, while in the first lowered position, substantially below the rooftop""s surface, and adapted for locking into place upon reaching the second position.
By way of example only to illustrate prior attempts to address rooftop damage: (a) a baffle stated to exert a displacing effect on wind flow across a roof surface is suggested in U.S. Pat. No. 4,005,557 issued to Kramer, et al. on Feb. 1, 1977xe2x80x94in Col. 3 of Patent ""557, the inventor states that xe2x80x9c[t]he optimum value of the ratio of open surface to closed surface for preventing or reducing wind suction forces should not exceed 50%xe2x80x9d, and (b) a wind spoiler ridge row cap for shallow pitched gabled roofs is suggested in U.S. Pat. No. 5,918,423 issued to Ponder on Jul. 6, 1999.